From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Journal page discussing voltage regulation and automotive electrical systems, including shunt-wound and compound-wound generators.
| Identifier | ExFiles\Box 31\1\ Scan172 | |
| Date | 1st June 1925 | |
| Vol. XVI June, 1925 No. 6 374 THE JOURNAL OF THE SOCIETY OF AUTOMOTIVE ENGINEERS use, motorcoach builders naturally applied it for use with heavy continuous loads where the conditions were entirely different from those usually encountered in the field of operation of passenger cars. The tendency for the third-brush system in continuous service is to charge at too high a rate when the battery is well up in charge, and to furnish insufficient energy when the battery is in a discharged condition. These facts were soon emphasized in the motorcoach installations, where the driving schedules vary from slow speed and frequent stops, in city service, to the fairly constant, high-speed continuous schedules on interurban and cross-country routes. It was soon recognized that some other means of controlling the output of the electric generator on motorcoaches is imperative if all their exacting requirements are to be met satisfactorily. The problem is being solved by the use of the voltage-regulated generator, which furnishes a controlled voltage for lamp lighting and results in a tapering battery-charging rate. Fig. 2 shows the voltage-regulated charging-characteristic, at constant speed, for a large battery. A somewhat popular misconception with reference to voltage-regulated systems prevails. The purpose of such an electrical system is to furnish a regulated potential within the limits required to give steady, non-flickering, non-changing light and to assure proper battery charging at the same time. Such systems are not “constant-potential” systems, in that the voltage is not held absolutely at one value, but varies slightly within the limits required by the application. While the motorbus is probably immediately responsible for the present demand for and interest in voltage-regulated generators, one manufacturing company pioneered in this development during the war when it produced voltage-regulated generators for lighting the sights of anti-aircraft gun-carriages. These guns were mounted on trailers and were parked in position. There was no engine to drive a generator, and no battery was available. No means were provided for charging batteries in the field. This generator was hand driven and maintained the steady light required, without flickering, for the accurate reading of various vernier scales; at the same time, it did not endanger the lamps from excessive voltage. It was a self-regulating unit, without a battery. Its advantages were soon appreciated by aeronautical engineers and it was applied to airplanes, with some refinements, so that the radio apparatus is now operated from the system, this being a very exacting requirement. Army airplanes of all types and the night flying airplanes of the United States Air Mail Service are equipped with similar apparatus. The round-the-world flyers used such units on their epoch-making flight. VOLTAGE REGULATION The subject of voltage regulation divides itself properly into two divisions, (a) constant-voltage systems and (b) voltage-regulated systems. By “constant voltage” is meant a practically unchanging terminal potential within very close limits, say 0.3 volt or less, as in the case of airplane radio installations, under varying conditions of load, speed and wide extremes of temperature, such as may be encountered in the air. “Voltage regulation,” as commonly applied, refers to a system that regulates within somewhat wider limits, such as those required on a motorbus or a motor truck for battery charging and lighting service where the main requirements are to keep the battery charged and to supply sufficient, steady light without flicker. Immediately the manufacturer is circumscribed by limiting conditions. On a 12-volt system, the lamps should be operated at approximately 14.5 volts for maximum life, and yet a voltage of 15 volts and sometimes more is required to charge the batteries used. Obviously, a compromise generator voltage is necessary. AUTOMOTIVE ELECTRICAL SYSTEMS A very brief history of automotive electrical systems, leading up to the development of the voltage-regulated system, is of interest. In general, the shunt-wound generator is best suited to automotive requirements, because of its inherent characteristics. The series and the compound-wound generators, two other well known types of direct-current machine, have series windings, and this fact alone makes them generally unsuitable. In a simple shunt-wound generator, the field winding consists of a comparatively large number of turns of small-sized wire. This winding is connected directly across the armature, being thus in parallel with the armature and with the external circuit. As a result of this arrangement, variations in the current in the external circuit do not react directly on the voltage. Such variations have a secondary effect and, within the working range of industrial operations, the shunt-wound machine is relatively a constant-potential generator. Usually, however, it is driven at somewhere near constant speed. An increase in the external current causes an increase in the armature current. The armature current is the sum of the external current and the small shunt-field current. As the armature current increases, the voltage drop in the armature is increased and the terminal voltage falls. The terminal voltage is therefore greatest at no load, and gradually falls off with increased load. Since the field winding is connected directly across the brushes, the drop in the terminal voltage causes a decrease in the shunt field-current that tends to diminish the field flux, and a smaller electromotive force is generated in the armature; so, the terminal voltage falls still lower. Compound-wound dynamos can be regarded as shunt dynamos, the fields of which are strengthened when the load comes on by a few series turns of wire carrying the main current; but these are entirely unsuitable for battery charging. If used for this purpose and the engine speed decreases, the counter electromotive force of the battery may overcome the electromotive force of the generator; then, the generator would be no longer in opposition, but the battery and the generator would act together in a circuit of low resistance, causing a dangerously high current to result. If a shunt-wound generator is used, the positive terminal of the battery should always be connected to the positive brush of the generator, and the shunt current in the field coils is always in the same direction, even when the main current is reversed. The electromotive forces of the battery and the generator are always opposed. When the main current reverses, the battery drives the generator as a motor, and the current cannot reach a dangerously high limit. As to the effect of speed variation, assuming the load to be constant, if the generator is running without a battery in parallel a change in speed has a marked effect on the terminal voltage. When in parallel with a battery, a change in the electromotive force of the generator will cause a change in the armature current. If the speed drops, the armature electromotive force decreases and, with it, the armature current. Since the current in the external circuit is constant, the battery, if discharging, must give a greater current and the smaller the | ||